The present invention relates to a semiconductor device, a semiconductor light emitting device and a method of manufacturing a semiconductor, and more particularly, it relates to a light emitting diode and a laser diode including a group II-VI compound semiconductor and a method of manufacturing a semiconductor used in such a semiconductor device.
A group II-VI compound semiconductor is regarded as a promising material for realizing an optical device dealing with short wavelength light. An initial semiconductor laser diode using a group II-VI compound semiconductor includes a metal electrode formed by directly evaporating a metal on p-type ZnSe (Appl. Phys. Lett. 59, 1991, p. 1272). However, this metal electrode has a large Schottky barrier between the metal included in the metal electrode and p-type ZnSe, and hence, the operating voltage of the resultant laser diode is disadvantageously high.
In order to overcome this disadvantage, a pseudo-graded superlattice structure including ZnSe and ZnTe, in which thin films of p-type ZnSe and p-type ZnTe are alternately stacked between the p-type ZnSe and the metal electrode so as to vary the average composition, is used as a contact layer. Thus, the operating voltage can be effectively decreased when a structure as designed is realized (for example, J. Vac. Sci. Technol. B 12(4). Jul/Aug 1994, pp. 2480-2483).
However, although a conventional laser device having the longest lifetime includes the aforementioned superlattice structure as a contact layer, its operating voltage is as high as 11 V (Electronics Letters Mar. 14, 1996, Vol. 32, pp. 552-553). This fact reveals that it is difficult to form a superlattice contact layer in an ideal structure as designed. When continuously operated at room temperature, even this laser device with the longest lifetime is broken in at most approximately 100 hours. The main cause of the breakdown seems to be low reproducibility of the contact layer having the superlattice structure.
The pseudo-graded superlattice structure includes plural stacked layers formed by repeating epitaxial growth of a short period of time of ZnSe and ZnTe, which have lattice constants different from each other by 8%. Accordingly, highly accurate epitaxial growth technique is required for attaining a structure as designed, and the work requires high skill. Also, the characteristic of a resultant contact layer is very sensitive to variation in the thickness of respective layers and a concentration of nitrogen used in p-type doping. In particular, the reproducibility of the current-voltage characteristic is low, and it is difficult to obtain a contact layer with a sufficiently low contact voltage.
Moreover, since there is a large lattice mismatch between ZnSe and ZnTe, the quality of the resultant crystal is poor. Therefore, not only a large number of crystal defects are caused during the growth but also strain remains within the contact layer after the growth. Accordingly, the device has poor stability against a heat treatment and current injection, resulting in easily degrading in the characteristic thereof.
Also, the contact layer has another problem that the resistance of a ZnSe layer is increased owing to diffusion of nitrogen. Since a ZnTe layer incorporates a much larger amount of nitrogen than a ZnSe layer during the growth, the nitrogen included in the ZnTe layer is diffused into the ZnSe layer through temperature increase or the like during or after the growth. As a result, the diffused nitrogen compensates or deactivates an acceptor of the ZnSe layer, and hence, the resistance of the ZnSe layer can be easily increased.
As described so far, the pseudo-graded superlattice structure has a large number of problems in difficulty in the manufacture, low reproducibility and instability in the operation.
An attempt has been made to provide an alloyed crystal layer of ZnSTe between a ZnTe layer and a ZnSe layer as a contact layer replaceable with the pseudo-graded superlattice structure (Appl. Phys. Lett. 70(10), 1997, pp. 1281-1283). However, in this structure, a large potential barrier is formed on an interface between the ZnSe layer and the ZnSTe layer due to discontinuity in their valence band. As a result, this structure cannot provide an ohmic electrode with a low resistance.
Alternatively, usage of a pseudo-graded superlattice structure including stacked layers of BeTe and ZnSe as a contact layer has been proposed (Appl. Phys. Lett. 64(16), 1994, pp. 2148-2150). In this structure, the lattice mismatch can be made smaller as compared with that in the pseudo-graded superlattice structure of ZnSe/ZnTe, but it is still necessary to control the thicknesses in an atomic layer order of respective layers by repeating epitaxial growth of a short period of time. Therefore, the crystal growth is complicated, and it is difficult to obtain a contact structure of a high quality at high reproducibility.
As described above, in the conventional semiconductor device of a group II-VI compound semiconductor, an appropriate contact layer to be formed between a group II-VI compound semiconductor and a metal electrode has not been found, and it has been difficult to manufacture a semiconductor device having a low operating voltage and high stability at high reproducibility.